In one type of thermal printer which prints colored images, a carrier contains a repeating series of spaced frames of difference colored heat transferable dyes. In such apparatus, the carrier is disposed between a receiver, such as coated paper, and a print head formed of, for example, a plurality of individual heating elements. When a particular heating element is energized, it is heated and causes dye from the carrier to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the carrier.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable dye density image pixel on the receiver.
The carrier often includes a repeating series of spaced yellow, magenta and cyan dye frames. First, the yellow frame and the receiver are moved to be positioned under the print head and as they are advanced, the heating elements are selectively energized to form a row of yellow image pixels in the receiver. This process is repeated until a yellow dye image is formed in the receiver. Next, the magenta frame is moved under the print head and the receiver is also moved under the print head. Both the receiver and the magenta frame are moved as the heating elements are selectively energized and a magenta image is formed superimposed upon the yellow image. Finally, as the cyan dye frame and the receiver are moved under the print head, the heating elements are selectively energized and a cyan dye image is formed in the receiver superimposed upon the yellow and magenta dye images. These yellow, magenta and cyan dye images combine to form a colored image. It has been found preferable to print the dye images in the order listed above; yellow, magenta and cyan. This results in the highest quality colored image being formed in the receiver.
Since the carrier has a repeating series of yellow, magenta and cyan dye frames, it is important to identify the leading yellow frame of each series. One way to identify the leading yellow frame is to employ a sensitometer. This sensitometer identifies a yellow dye frame by producing a particular analog signal in response to light which passes a yellow dye frame. Such sensitometer is effective but can be a complex and expensive piece of equipment. Another way to identify a yellow dye frame is to provide code marks. A code field composed of a series of black code bars can be disposed in the clear interframe area between dye frames. This code field can identify the particular color of the following frame. A reader station can be provided which includes a plurality of photodetectors which are aligned to produce a particular output signal representing the color of the following colored frame. Such a system can perform quite satisfactorily but requires complete decoding electronics and involves some manufacturing complexity for forming the code field in the clear interframe areas.